The present invention relates to a positron imaging device for attaining information such as the distribution of matter in a measured subject.
A positron imaging device is that which attains information on a measured subject, such as a human body, an animal, or a plant, by the injection as a tracer of a material marked with radioactive isotopes (RI) for emitting positrons, and measuring gamma ray pairs generated by the pair annihilation of the positrons emitted by the RI material with electrons present in normal matter (for example, see Japanese Patent Application Laid-open No. H09-33658).
A gamma ray pair generated by the electron-positron pair annihilation has energy approximately equivalent to the mass of the electron or positron (511 keV) and the rays are emitted in mutually opposite directions. Consequently, it becomes possible to measure the distribution of matter or the like at each position in the measured subject by coincidently counting the gamma ray pairs with radiation detectors disposed with the measured subject therebetween and identifying the locations at which pair annihilation occurs.
Devices with a number of constitutions such as the positron CT device have been proposed or developed as positron imaging devices. One such device detects gamma ray pairs with a pair of two-dimensional radiation detectors disposed opposite to each other. This type of device is used for the measurement of relatively flat measured subjects placed at a position between the two two-dimensional radiation detectors, for example, such as the measurement of plant materials. It is also possible for such a device to be used for items with thickness.
FIG. 8 shows the basic constitution of this type of positron imaging device. This device comprises a pair of two-dimensional radiation detectors 60 and 70. The two-dimensional radiation detectors 60 and 70 each comprise scintillator arrays 61, 71 comprising a plurality of scintillators, and photodetectors 62, 72 for detecting the scintillation light generated by gamma rays incident on the scintillator arrays 61, 71. The radiation detectors 60 and 70 are disposed within detector cases 65, 75 so that the radiation incident surfaces of the scintillator arrays 61, 71 are opposite to each other.
In this constitution, the pair of gamma rays emitted from the measured subject A disposed on a prescribed measurement surface S between the radiation detectors 60 and 70 is detected by both the radiation detectors 60 and 70. Those detection signals are output through circuit systems 63, 73, each comprising an amp circuit or the like for amplifying the detection signal. The detection signal output is input to a signal processing circuit 8 and the signal processing circuit 8 specifies the electron-positron pair annihilation event by coincidence counting based on the detection signals from the radiation detectors 60 and 70 and performs calculations and so forth of the position at which the pair annihilation occurred.
For the positron imaging device with the constitution discussed above, the area of the field of view must be expanded in order for the efficient measurement of various measured subjects. In the constitution of the device with the mutually opposite radiation detectors, the area of the region of the radiation incident surface, in each of the mutually opposite radiation detectors, becomes the area of the field of view that can be measured.
FIG. 9 is a side view showing the expansion of the range of the field of view in the case of two pairs of opposite radiation detectors. This imaging device has a measurement surface S, whereon the measured subject such as a plant is disposed, held between two opposite detector groups 6 and 7. The detector groups 6 and 7 are each constituted by two radiation detectors 601, and 602and 701and 702. In this constitution, the range of the field of view Ve is formed from the opposite radiation detectors 601and 701and the range of the field of view Vf is formed from the radiation detectors 602 and 702. As shown with a plan view of this range of the field of view on the right end of the figure, double the range of the field of view V is attained as a whole, as opposed to the constitution using only one pair of detectors.
The expansion of the abovementioned range of the field of view is equivalent to the case of simply disposing two adjacent imaging devices each comprising one pair of radiation detectors. The efficiency of this expansion of the field of view is poor. In particular, in such a method for expanding the range of the field of view, the number of radiation detectors disposed increases in proportion to the area of the field of view to be attained. The increased cost of the device with the expansion of the field of view becomes therefore problematic.
The present invention was developed in view of the abovementioned problems and it is an object of the present invention to provide a positron imaging device wherein the range of the field of view is efficiently expanded, while the constitution of the device is simplified and the costs are reduced.
In order to achieve this object, the positron imaging device according to the present invention is a positron imaging device for attaining an image of a measured subject by coincidence counting of a gamma ray pair generated by electron-positron pair annihilation in the measured subject and emitted in mutually opposite directions; and this device comprises: (1) a first detector group having a prescribed number, that is two or more, of radiation detectors including radiation detecting portions constituted so that two-dimensional position detection is possible, in which each of the radiation detectors is arranged so that the radiation detecting portion is disposed with an arrangement spacing from the radiation detecting portion of the radiation detector adjacent thereto; (2) a second detector group having the prescribed number of the radiation detectors and in which each of the radiation detectors is arranged so as to be opposite the corresponding radiation detectors of the first detector group; and (3) signal processing means to which are input a first detection signal output from the first detector group and a second detection signal output from the second detector group, and for carrying out coincidence counting of the first detection signal and the second detection signal; (4) wherein the signal processing means carry out coincidence counting of the first detection signal output from each of the radiation detectors constituting the first detector group with the second detection signals output from a plurality of the radiation detectors including the opposed radiation detector from among the radiation detectors constituting the second detector group.
In the abovementioned imaging device, the two detector groups disposed opposite to each other are constituted from a plurality of two-dimensional radiation detectors and are disposed so that the radiation detectors in the detector groups have spacing between the detecting portions. Furthermore, the signal processing means for carrying out coincidence counting for the two detector groups are constituted so as to perform coincidence counting for the diagonally disposed detectors as well as the detectors disposed directly opposite to the detectors constituting each of the detector groups.
At this time, the area between the detecting portions becomes the range of the field of view resulting from coincidence counting for the detectors disposed directly opposite from each other. Meanwhile all or part of the area between the detecting portions arranged with spacing therebetween supplemented by coincidence counting for the detectors disposed diagonally to each other and becomes the range of the field of view. Consequently, it becomes possible to supplement the area between detectors and realize an efficient expansion of the range of the field of view with a detector arrangement including spacing and by coincidence counting for oppositely and diagonally disposed detectors. At this time, a greater rate of expansion of the range of the field of view is attained than a rate of increase of the detectors.
Because of problems in the constitution of two-dimensional radiation detectors, spacing necessarily is formed between the detecting portions when a plurality of detectors is arranged. However, in this case as well, it becomes possible to supplement the insensitive region, due to the spacing between the detecting portions, and have a measurable range of the field of view with the device constitution discussed above.
Moreover, for the detector arrangement with spacing, various arrangements may be used and the arrangement spacing need not be uniform for the entire detector group.